The present invention is concerned with the sound damping of noise-producing engines such as internal combustion engines that are housed in a bonnet substantially completely enclosing the engine and the cooling of which is provided by a fan associated with a radiator. It is applicable particularly to public works or agricultural machines of the type in which an engine, usually a diesel engine, is housed in a bonnet which encloses it substantially completely and which has inlet and outlet orifices or openings for the cooling air. It is applicable more particularly to power units or machines provided with a blowing fan which discharges cooling air from the engine to the outside of the bonnet through a radiator traversed in the conventional way by the cooling liquid.
It is known that machines such as public works machines are the source of very high noise levels, and the regulations in force in most countries limit the noise level which is permissible for such machines which often have to operate not far from residential localities. Consequently, numerous solutions have already been proposed for reducing this noise level by the interposition between the source of the noise and the outside environment and absorbing screens provided with openings for the passage of air for cooling the power unit.
In the prior technology, it has thus been proposed (French Pat. No. 73-37 336) to mount, in the outlet path of air discharged by a blower through a radiator fitted to an engine of a public works machine, a sound damping assembly having non-rectilinear passages delimited by strips coated with foam material which ensures the absorption of noise, the non-rectilinear shape of these strips preventing direct outflow of the air and hence a likewise direct radiation of the sound waves. But such a solution, even if it has certain advantages, involves a relatively complex and expensive constructional arrangement and it is combersome, which may necessitate a lengthening of the frame of the machine in order to support the sound damping assembly.
It has moreover been proposed in to mount, in German Patent Application No. 25 54 750 the outflow path of the air discharge by a blower through a radiator, noise-reducing dampers protected by a perforated metal sheet. But in this case also, the cumbersomeness of the sound damping system is relatively great and the arrangement as a whole is a rather complex construction.
It has also been proposed, for the sound damping of public works or agricultural machines (French Pat. No. 81-05 510) to interpose, in the outflow path of the air discharged by the blower, a block of cellular material or foam material provided with parallel channels for the passage of air having dimensions included within certain limits. The bulkiness of such an acoustic damping arrangement is therefore distinctly more reduced and the carrying out of the arrangement is simple.